1. Field of the Disclosure
Embodiments disclosed herein relate generally to a process for the reduction or removal of benzene in a hydrocarbon stream, such as a gasoline fraction. More specifically, embodiments disclosed herein relate to the removal of benzene from a reformate stream.
2. Background
One common process long used by the refinery industry to upgrade raw naphtha to high octane gasoline is catalytic reforming. In catalytic reforming the raw naphtha having a boiling range from about 46 to 177° C. (115° F.-350° F.) is passed over an alumina supported noble metal catalyst at elevated temperatures (about 493° C.-565° C. (920° F.-1050° F.)) and moderate pressure (about 2 bar to 39 bar (about 15-550 psig)). The catalyst “reforms” the molecular structures of the hydrocarbons contained in the raw naphtha by removing hydrogen and rearranging the structure of the molecules so as to improve the octane number of the naphtha. However, the increase in octane number also reduces the liquid volume of the naphtha as the specific gravity is increased.
Because of the multiplicity of the compounds in the raw naphtha, the actual reactions which occur in catalytic reforming are numerous. However, some of the many resulting products are aryl or aromatic compounds, all of which exhibit high octane numbers. The aryl compounds produced depend upon the starting materials which in a refinery are controlled by the boiling range of the naphtha used and the crude oil source.
The “reformed” product from a catalytic reforming process is commonly called reformate and is often separated into two fractions by conventional distillations—a light reformate having a boiling range of about 46° C.-121° C. (about 115° F.-250° F.) and a heavy reformate having a boiling range of about 121° C.-177° C. (about 250° F.-350° F.). The aryl compounds in each fraction are thus dependent upon their boiling points. The lower boiling or lighter aryl compounds, e.g., benzene, toluene and xylenes, are contained in the light reformate and higher boiling aryl compounds are contained in the heavy reformate. In other circumstances, the light reformate may contain only the benzene, or only benzene and toluene, depending upon any downstream processing of the stream.
The demand for cleaner and safer transportation fuels is becoming greater every year. Two major sources of gasoline feedstock, including reforming and catalytic cracking, present both a problem meeting strict environmental regulations and impose certain health risks. For example, light reformate typically contains unacceptably high levels of benzene, a known carcinogen.
Refiners in the U.S. and in other countries are required to remove benzene from reformate streams and other gasoline fractions. Various options for the removal of benzene from such streams may include distillation, extraction, hydrogenation, alkylation, and transalkylation. However, due to a limited quantity of benzene contained in non-reformate streams, it may be uneconomical for a non-integrated refiner to recover benzene from various gasoline fractions, such as naphtha produced in a fluid catalytic cracking (FCC) unit, for example.
Extraction of benzene requires expensive capital investment in necessary equipment and a customer for the benzene product, neither of which may be feasible for a small, non-integrated refiner. Also, while it is possible to extract benzene from the gasoline pool by fractionation techniques, such techniques are not preferred, because the boiling point of benzene is too close to that of some of the more desirable organic components, including C6 paraffins and isoparaffins. Monoalkylate aromatics (monoalkylate), such as toluene, xylenes, and ethylbenzene are more desirable for gasoline blending, as opposed to benzene, because they are less objectionable both from an environmental and a safety point of view. Additionally, toluene, xylenes, and ethylbenzene each have a higher octane rating than benzene.
Alternatively, benzene in reformate may be removed via hydrogenation. However, non-selective hydrogenation of aromatics, such as benzene and toluene in a reformate stream, results in reduced octane rating and thus diminishes the overall value of the fuel.
Generally refiners tend to prevent benzene from entering the gasoline blending stock. For example as mentioned above the light reformate may be subjected to aromatic removal by solvent extraction. This, however, removes all aromatic material not just the benzene. One method of preventing the introduction of benzene into the gasoline pool is to remove the benzene precursors (methyl cyclopentane and isohexane) from the charge to the catalytic reforming units. This does not solve the problem of streams which contain benzene as well as heavier aromatic compounds such as toluene and xylenes. The heavier aromatics contribute greatly to the octane pool and to date have not been found to be detrimental to the environment.
U.S. Pat. No. 5,773,670 discloses a process for the hydrogenation of aromatics in a petroleum stream. However, like solvent extraction, the process is not selective to only the benzene. U.S. Pat. No. 5,856,602 discloses the hydrogenation of aromatics in a hydrocarbon stream utilizing a distillation column reactor wherein the placement of the catalyst bed and operation of the distillation column controls which aromatic is retained in the catalyst bed for hydrogenation. U.S. Pat. No. 6,187,980 discloses a process for the hydrogenation of benzene to cyclohexane in a distillation column reactor wherein essentially pure benzene is used as the feed to the reactor.
Benzene hydrogenation in U.S. Pat. No. 5,856,602 may be conducted at disclosed pressures of less than about 120 psig, and temperatures of about 65° C. to 204° C. (150° F. to 400° F.). Such a process may allow the separation of a benzene concentrated stream from reformate and selectively hydrogenating the benzene in a single unit. In such a unit, the activity of the catalyst can be increased by increasing the column operating pressure to gain temperature and partial pressure of hydrogen. Such an increase in catalyst activity may be desired to meet increasingly stringent requirements on gasoline benzene concentration, for example.
Unfortunately, operation of a column at the higher pressures requires higher reflux as the relative volatility between benzene and toluene becomes smaller. Too high a pressure will also cause high temperatures in the column bottoms, and may require a fire heater. Another disadvantage is that the hydrogen fed to the column will be diluted by the vapor rate required for the higher reflux rate. These disadvantages may cause an increase in the catalyst requirements, and the catalyst requirements may be significant, especially where the requirement of benzene is less than 0.1 or 0.2 weight percent.
Accordingly, there is still a significant need in the art for economical methods to reduce the levels of benzene in refinery streams, such as to below 0.1 or 0.2 weight percent, without using an excessive amount of catalyst.